The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) from the powertrain to all four wheels of the vehicle. In many power transfer systems, a power transfer unit, such as a transfer case, is incorporated into the driveline and is operable for delivering drive torque from the powertrain to both the front and rear wheels. Many conventional transfer cases are equipped with a mode shift mechanism that can be selectively actuated to shift between a two-wheel drive mode and a four-wheel drive mode.
It is also known to use “on-demand” power transfer systems for automatically biasing power between the front and rear wheels, without any input or action on the part of the vehicle operator, when traction is lost at either the front or rear wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is typically maintained in a non-actuated condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the clutch assembly is automatically actuated to deliver torque “on-demand” to the front wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-slipping wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. This on-demand clutch control system can also be used in full-time transfer cases to automatically bias the torque ratio across an interaxle differential.
Notwithstanding significant sales of four-wheel drive and all-wheel drive vehicles, much emphasis is directed to improving vehicle performance and fuel efficiency while at the same time reducing weight. In conflict with this emphasis is the need to engineer the components of power transfer units to meet all torque requirements anticipated for the vehicle application. Specifically, the components must be sized to survive during torque peak conditions despite the fact that such peak conditions rarely occur during typical use of the motor vehicle. Thus, a need exists to limit the maximum torque transferred by a power transfer unit so as to permit the components to be smaller in size and weight.